From: Job Snijders <job@sobornost.net>
Date: Tue, 25 Jun 2024 05:21:39 +0000 (+0000)
Subject: Generate all permutations of the list with equal probability
X-Git-Tag: 1.6.3~13
X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=7b72fe8a2bddc5134c821bbe58ef35e50a22cf1e;p=thirdparty%2FFORT-validator.git

Generate all permutations of the list with equal probability

@botovq was kind enough to point out that although my earlier
implementation produced random-ish ordering, it strictly speaking
wasn't Fisher-Yates.

We need to ensure `j` is a random number between `i` and `list.count`
see the second example in the 'Modern Algorithm'
https://en.wikipedia.org/wiki/Fisher%E2%80%93Yates_shuffle
---

diff --git a/src/object/manifest.c b/src/object/manifest.c
index a50e2889..7f188e2e 100644
--- a/src/object/manifest.c
+++ b/src/object/manifest.c
@@ -205,8 +205,8 @@ build_rpp(struct Manifest *mft, struct rpki_uri *notif,
 
 	/* Fisher-Yates shuffle with modulo bias */
 	srand(time(NULL) ^ getpid());
-	for (i = 0; i < mft->fileList.list.count; i++) {
-		j = rand() % mft->fileList.list.count;
+	for (i = 0; i < mft->fileList.list.count - 1; i++) {
+		j = i + rand() % (mft->fileList.list.count - i);
 		tmpfah = mft->fileList.list.array[j];
 		mft->fileList.list.array[j] = mft->fileList.list.array[i];
 		mft->fileList.list.array[i] = tmpfah;